This invention relates to a digital color correction method which separates and corrects the colors of an original color image by photo-electronic scanning and then outputs picture image signals for reconstructing a color picture image and the apparatus therefor.
In the color tone correction generally conducted by scanners, etc., in addition to the basic masking for eliminating improper absorption of ink colors, there is provided means for removing distortion of colors or for slightly correcting the tone of a particular hue according to the taste of an individual operator. Such a correction is achieved by discriminating the hue and saturation of an original image with the basic three color signals and by adding appropriate correction signals to the basic signals. FIG. 1 shows an example of the structure disclosed in Japanese Kokoku No. 50-14845 wherein the color separation signals B, G, R, obtained by photo-electronic scanning using color separation filters of blue (B), green (G) and red (R), are converted to density signals Y.sub.0, M.sub.0, C.sub.o in logarithmic conversion circuits 1 through 3. The density signals Y.sub.0, M.sub.0, C.sub.0 are fed to masking circuits 4 through 6 and converted to the corrected density signals Y.sub.1, M.sub.1, C.sub.1. The masking circuits 4 to 6 perform computations such as Y.sub.1 =Y.sub.0 -aM.sub.0 -bC.sub.0 supply corrected density signals Y.sub.1, M.sub.1, C.sub.1 to a color correction signal generating circuit 7 which outputs color correction signals Y.sub.C, M.sub.C, C.sub.C which correspond to density signals Y.sub.1, M.sub.1, and C.sub.1. The color correction signal Y.sub.C is, for example, of a form of Y.sub.C =a1.multidot.(Y)+a2.multidot.(G)+a3.multidot.(C)+a4.multidot.(B)+a5.multid ot.(M)+a6.multidot.(R). The other color correction signals M.sub.C and C.sub.C are of a similar form. The symbols (Y), (G), . . . , (R) denote the hue signals formed in the color correction signal generating circuit 7. The color correction signals Y.sub.C, M.sub.C, C.sub.C are respectively inputted to the subtracting terminals of subtracters 8 through 10 and the density signals Y.sub.1, M.sub.1, C.sub.1 from the masking circuits 4 through 6 are respectively inputted to the adding terminals thereof; the subtracters 8, 9 and 10 output signals Y.sub.2 =Y.sub.1 -Y.sub. C, M.sub.2 =M.sub.1 -M.sub.C, C.sub.2 =C.sub.1 -C.sub.C, respectively. The coefficients a, b mentioned above may be arbitrarily varied by a potentiometer, etc., and the coefficients a1 through a6 are also arbitrarily varied. These coefficients are adjusted by an operator whenever necessary. The description of black print signal circuits for producing a black print is omitted for simplifying the explanation.
The correction system of the prior art mainly aims at eliminating incompleteness which means that the spectral characteristics of the inks overlap to a considerable extent. The masking circuits 4 through 6 attain such an object, being supplemented by an operator of the scanner who sets coefficients appropriately for producing prints with an accurate color rendition. The system therefore does not consider the correction of improperly absorbed components of color elements in an original image which are contained in the density signals Y.sub.0, M.sub.0, C.sub.0 obtained by photo-electronic scanning of the color image. Improperly absorbed components of color elements refer to those color elements having undesired spectral transmittances (e.g.--color dyes, color pigment inks, etc.). It is impossible to know the exact amount of these color elements. The weights of respective components in the corrected density signals Y.sub.1, M.sub.1, C.sub.1 which are to be inputted at the color correction signal generating circuit 7 are not always equal. Hue signals (Y), (G), (C), (B), (M), (R) are produced from the color correction signal generating circuits 7 based on the density signals Y.sub.1, M.sub.1, C.sub.1. When a neutral color portion of an original image is scanned, the hue signals of all the 6 types should be zero. But if the weight of the density signals Y.sub.1, M.sub.1, C.sub.1 is not equal, either one of the hue signals with a positive value is outputted and a correction signal implying as if the image is colored may be outputted accidentally. There cannot be more than two hue signals with a positive value. In other words, as the system conducts computation using symmetrical formulas on the 3 color signals, if the weight of respective signals is not equal, the image is discriminated to be a hue different from the actual hue, especially near the neutral color portion (gray), thereby producing undesirable tones in the reproduced picture image. When a noise or an error occurs in the color signal in the process prior thereto, a similar phenomenon may occur.
The hue signals (Y), (G), . . . , (R) used may be obtained by computation using the basic three color signals. Since the color in the original image belongs to one of the six hues of blue, cyan, green, yellow, red and magenta which are obtained by dividing a color space into six portions, the computation of one hue signal or the computation of at most two hue signals adjacent to the color of the original image would suffice. This relationship is shown in FIG. 3. However, since it is impossible to decide before computation to which hue it should be classified, all of the hue signals must be computed. If, in order to carry this out in an analog circuit, the number of circuits corresponding to the number of hues should be prepared so as to conduct parallel computation and circuits should be adjusted; this is not only troublesome but is likely to cause greater errors. If such a system is constructed exclusively by digital circuits, on the other hand, the space required by the circuits for parallel computation becomes large, pushing up the cost. Although hue signals may be computed by digital circuits in time sequence, the speed is slow compared to the parallel computation.
There has been proposed a circuit using digital operation in order to avoid the temperature dependency or chronological changes of analog circuit components which used to be utilized in the prior art color correction circuits. For instance, as disclosed in Japanese Laid-open Patent Application No. 123201/1978, for executing high-speed real time processing, a memory table of the outputs Y.sub.2, M.sub.2, C.sub.2 corresponding to the input density signals Y.sub.0, M.sub.0, C.sub.0, and interpolation are employed to avoid a large capacity memory. However, if input data is used as address as a means of digitalizing color computation and a memory table which has been written-in with output data is accessed, the memory table has to be prepared prior to actual scanning. Further, it is inconvenient in that the output data Y.sub.2, M.sub.2, C.sub.2 must be computed for all the combinations of the input density signals Y.sub.0, M.sub.0, C.sub.0. When coefficients which used to be set by a potentiometer in the analog circuit had to be varied, those computations must be redone for all the memory tables. Such procedures are not only complicated but might require operations extremely different from the conventional method. Even if the system is structured to divide the color computation into plural steps, to prepare such memory tables as mentioned above on each step, and to re-write only the tables which are related to the change of coefficients would not achieve the effectiveness which justifies all the trouble. If interpolation is employed in order to reduce the memory capacity, there runs a higher possibility of making output data unnatural by errors in computation. In order to avoid such mistakes, more complicated interpolation will be additionally needed, thereby incapacitating real time processing.